1. The Ultimate X-Ray
2. Fake Calls

It imitates you receiving a call. You can pick out who is calling, delay when the ???call??? actually occurs. Have President Obama give you a call requesting your help on a CIA mission.
3. Pranktionary

A $0.99 iPhone app that can randomly suggest pranks for the home, the office, or for that lovely date you??™re going on tomorrow.
4. Mister Prank

Mister Prank is a simple iPhone application that mimics the functionality of online soundboards. It has a series of pre-recorded phrases to use in lie of you talking on the phone.
5. iFart
Jeff Rickard on April 3rd, 2009 at 20:17 says:
There are more apps than this out there, if you are looking for the soundboard functionality, I recommend BeefyBoard, simply because you can download more sounds into it as you need them and you can switch out the boards to your hearts content.